Silica and also epoxy resins

Abstract

Fumed silicon dioxide powder in the form of aggregated primary particles having a BET surface area of 175±15 m 2 /g and a thickening effect, based on the BET surface area, of greater than 18 mPas g/m 2 is hydrophobicized with polydimethylsiloxane. This hydrophobic fumed silica has a BET surface area of 110±25 m 2 /g. It can be used in epoxy resins. These epoxy resins in turn can be used as adhesives.

The invention claimed is: 
     
       1. A process for preparing a fumed hydrophilic silicon dioxide powder comprising aggregated primary particles having a BET surface area of 175±15 m 2 /g, wherein a thickening effect of the powder, based on the BET surface area, is greater than 18 mPas·g/m 2 , the process comprising:
 vaporizing a silicon compound; 
 transferring the vaporized silicon compound, optionally with a carrier gas, into a mixing chamber, 
 separately, transferring a fuel gas and primary air into the mixing chamber, and mixing with the silicon compound vapour; 
 igniting the mixture in a burner wherein a flame is burnt into a reaction chamber; 
 introducing secondary air surrounding the flame into the reaction chamber; 
 separating solid silicon dioxide from gaseous substances; and 
 treating the separated solid with a steam at 250° C. to 750° C., 
 wherein 
 the primary air is optionally preheated or enriched with oxygen, 
 a ratio of secondary air/primary air is from 0.05 to 3, 
 a total amount of oxygen is at least sufficient for the complete combustion of the fuel gas and the silicon compound, and 
 an adiabatic flame temperature T ad  is from 1840 to 1900° C., wherein T ad =temperature of feedstocks+sum of the reaction enthalpies of the sub-reactions/heat capacity of the substances which depart from the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, carbon dioxide, oxygen, nitrogen and, a carrier gas, if the latter is not air or nitrogen, on the basis of the specific heat capacity of these substances at 1000° C. 
 
     
     
       2. The process as claimed in  claim 1 , wherein the silicon compound is SiCl 4 , and the fuel gas is H 2 . 
     
     
       3. The process as claimed in  claim 2 , wherein for 100 kg/h of SiCl 4  transferred to the mixing chamber, 53 Nm 3 /h of H 2 , 155 Nm 3  of primary air and 55 Nm 3  of secondary air are employed. 
     
     
       4. The process as claimed in  claim 2 , wherein the secondary air/primary air ratio is 0.35. 
     
     
       5. The process as claimed in  claim 1 , wherein the thickening effect relative to the surface area of the powder is 20.08 mPas·g/m 2 . 
     
     
       6. A hydrophobic fumed silica, wherein
 a BET surface area is 110±25 m 2 /g, 
 an average aggregate surface area is 53 257±500 nm 2 , 
 an average aggregate circumference is 3605±500 nm, 
 an average maximum aggregate diameter is 394±5 nm, 
 an average minimum aggregate diameter is 242±5 nm, and 
 an average primary particle diameter is 18±5 nm, 
 wherein 
 the hydrophobic fumed silica is obtained by a process comprising: 
 spraying the hydrophilic fumed silica obtained by the process according to  claim 1  with polydimethylsiloxane, followed by mixing and then heat treatment for 1 to 6 hours. 
 
     
     
       7. The hydrophobic fumed silica according to  claim 6 , wherein the heat treatment is conducted at 360° C. for 3 hours under a nitrogen atmosphere. 
     
     
       8. An adhesive composition which comprises the hydrophobic fumed silica according to  claim 6  and an epoxy resin. 
     
     
       9. An epoxy resin composition comprising the hydrophobic fumed silica according to  claim 6 . 
     
     
       10. A method to adhere two surfaces comprising applying the epoxy resin composition according to  claim 9  to at least one of the two surfaces.